Ethylenediaminetetraacetic acid (EDTA), also known by several other names, is a chemical used for both industrial and medical purposes. It was synthetized for the first time in 1935 by Ferdinand Münz.[3]

It is an aminopolycarboxylic acid and a colourless, water-soluble solid. Its conjugate base is ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ("six-toothed") ligand and chelating agent, i.e., its ability to "sequester" metalions such as Ca2+ and Fe3+. After being bound by EDTA into a metal complex, metal ions remain in solution but exhibit diminished reactivity. EDTA is produced as several salts, notably disodium EDTA and calcium disodium EDTA.

The reduction of water hardness in laundry applications and the dissolution of scale in boilers both rely on EDTA and related complexants to bind Ca2+, Mg2+, as well as other metal ions. Once bound to EDTA, these metal centers tend not to form precipitates or to interfere with the action of the soaps and detergents. For similar reasons, cleaning solutions often contain EDTA. In a similar manner EDTA is used in the cement industry for the determination of free lime and free magnesia in cement and clinkers.[6][page needed]

The solubilization of ferric ions, at or below near neutral pH can be accomplished using EDTA. This property is useful in agriculture including hydroponics. However, given the pH dependence of ligand formation, EDTA is not helpful for improving Fe solubility in above neutral soils.[7] Otherwise, at near-neutral pH and above, iron(III) forms insoluble salts, which are less bioavailable to susceptible plant species. Aqueous [Fe(edta)]- is used for removing ("scrubbing") hydrogen sulfide from gas streams. This conversion is achieved by oxidizing the hydrogen sulfide to elemental sulfur, which is non-volatile:

In this application, the ferric center is reduced to its ferrous derivative, which can then be reoxidized by air. In similar manner, nitrogen oxides are removed from gas streams using [Fe(edta)]2-. The oxidizing properties of [Fe(edta)]- are also exploited in photography, where it is used to solubilize silver particles.[8]

EDTA was used in separation of the lanthanide metals by ion-exchange chromatography. Perfected by F.H. Spedding et al. in 1954, the method relies on the steady increase in stability constant of the lanthanide EDTA complexes with atomic number. Using sulfonated polystyrene beads and copper(II) as a retaining ion, EDTA causes the lanthanides to migrate down the column of resin while separating into bands of pure lanthanide. The lanthanides elute in order of decreasing atomic number. Due to the expense of this method, relative to counter-current solvent extraction, ion-exchange is now used only to obtain the highest purities of lanthanide (typically greater than 4N, 99.99%).[]

Dentists and endodontists use EDTA solutions to remove inorganic debris (smear layer) and lubricate the canals in endodontics. This procedure helps prepare root canals for obturation. Furthermore, EDTA solutions with the addition of a surfactant loosen up calcifications inside a root canal and allow instrumentation (canals shaping) and facilitate apical advancement of a file in a tight/calcified root canal towards the apex.

It serves as a preservative (usually to enhance the action of another preservative such as benzalkonium chloride or thiomersal) in ocular preparations and eyedrops.

EDTA is used extensively in the analysis of blood. It is an anticoagulant for blood samples for CBC/FBEs.[]

EDTA is a slime dispersant, and has been found to be highly effective in reducing bacterial growth during implantation of intraocular lenses (IOLs).[11]

Alternative medicine

Some alternative practitioners believe EDTA acts as an antioxidant, preventing free radicals from injuring blood vessel walls, therefore reducing atherosclerosis.[9] These ideas are unsupported by scientific studies, and seem to contradict some currently accepted principles.[12] The U.S. FDA has not approved it for the treatment of atherosclerosis.[13]

Cosmetics

In shampoos, cleaners, and other personal care products, EDTA salts are used as a sequestering agent to improve their stability in air.[14]

Laboratory applications

In the laboratory, EDTA is widely used for scavenging metal ions: In biochemistry and molecular biology, ion depletion is commonly used to deactivate metal-dependent enzymes, either as an assay for their reactivity or to suppress damage to DNA or proteins.[15] In analytical chemistry, EDTA is used in complexometric titrations and analysis of water hardness or as a masking agent to sequester metal ions that would interfere with the analyses. EDTA finds many specialized uses in the biomedical laboratories, such as in veterinaryophthalmology as an anticollagenase to prevent the worsening of corneal ulcers in animals. In tissue culture EDTA is used as a chelating agent that binds to calcium and prevents joining of cadherins between cells, preventing clumping of cells grown in liquid suspension, or detaching adherent cells for passaging. In histopathology, EDTA can be used as a decalcifying agent making it possible to cut sections using a microtome once the tissue sample is demineralised. EDTA is also known to inhibit a range of metallopeptidases, the method of inhibition occurs via the chelation of the metal ion required for catalytic activity.[16] EDTA can also be used to test for bioavailability of heavy metals in sediments. However, EDTA may influence the bioavailability of metals in solution, which may pose concerns regarding its effects in the environment, especially given its widespread uses and applications.

Side effects

EDTA exhibits low acute toxicity with LD50 (rat) of 2.0 g/kg to 2.2 g/kg.[8] It has been found to be both cytotoxic and weakly genotoxic in laboratory animals. Oral exposures have been noted to cause reproductive and developmental effects.[14] The same study by Lanigan[14] also found that both dermal exposure to EDTA in most cosmetic formulations and inhalation exposure to EDTA in aerosolized cosmetic formulations would produce exposure levels below those seen to be toxic in oral dosing studies.

This process is used to produce about 80 kilotonnes of EDTA each year. Impurities cogenerated by this route include glycine and nitrilotriacetic acid; they arise from reactions of the ammonia coproduct.[8]

Nomenclature

To describe EDTA and its various protonated forms, chemists distinguish between EDTA4-, the conjugate base that is the ligand, and H4EDTA, the precursor to that ligand. At very low pH (very acidic conditions) the fully protonated H6EDTA2+ form predominates, whereas at very high pH or very basic condition, the fully deprotonated EDTA4- form is prevalent. In this article, the term EDTA is used to mean H4-xEDTAx-, whereas in its complexes EDTA4- stands for the tetra-deprotonated ligand.

Coordination chemistry principles

Structure of [Fe(EDTA)(H2O)]-, showing that the EDTA4- ligand does not fully encapsulate Fe(III), which is seven-coordinate.[19]

In coordination chemistry, EDTA4- is a member of the aminopolycarboxylic acid family of ligands. EDTA4- usually binds to a metal cation through its two amines and four carboxylates. Many of the resulting coordination compounds adopt octahedral geometry. Although of little consequence for its applications, these octahedral complexes are chiral. The anion [Co(EDTA)]- has been resolved into enantiomers.[20] Many complexes of EDTA4- adopt more complex structures due to either the formation of an additional bond to water, i.e. seven-coordinate complexes, or the displacement of one carboxylate arm by water. The Fe(III) complex of EDTA is seven-coordinate.[21] Early work on the development of EDTA was undertaken by Gerold Schwarzenbach in the 1940s.[22] EDTA forms especially strong complexes with Mn(II), Cu(II), Fe(III), Pb(II) and Co(III).[23]

Several features of EDTA's complexes are relevant to its applications. First, because of its high denticity, this ligand has a high affinity for metal cations:

Written in this way, the equilibrium quotient shows that metal ions compete with protons for binding to EDTA. Because metal ions are extensively enveloped by EDTA, their catalytic properties are often suppressed. Finally, since complexes of EDTA4- are anionic, they tend to be highly soluble in water. For this reason, EDTA is able to dissolve deposits of metal oxides and carbonates.

Environmental fate

Abiotic degradation

EDTA is in such widespread use that questions have been raised whether it is a persistent organic pollutant. While EDTA serves many positive functions in different industrial, pharmaceutical and other avenues, the longevity of EDTA can pose serious issues in the environment. The degradation of EDTA is slow. It mainly occurs abiotically in the presence of sunlight.[24]

The most important process for the elimination of EDTA from surface waters is direct photolysis at wavelengths below 400 nm.[25] Depending on the light conditions, the photolysis half-lives of Fe(III) EDTA in surface waters can range as low as 11.3 minutes up to more than 100 hours.[26] Degradation of FeEDTA, but not EDTA itself, produces Fe complexes of ED3A, EDDA, and EDMA - 92% of EDDA and EDMA biodegrades in 20 hours while ED3A displays significantly higher resistance. Many environmentally abundant EDTA species (e.g., Mg2+, Ca2+) are more persistent.

Biodegradation

In many industrial wastewater treatment plants, EDTA elimination can be achieved at about 80% using microorganisms.[27] Resulting byproducts are ED3A and IDA - suggesting that both the backbone and acetyl groups were attacked. Some microorganisms have even been discovered to form nitrates out of EDTA but degrade optimally at moderately alkaline conditions of pH 9.0-9.5.[28]

Several bacterial strains isolated from sewage treatment plants efficiently degrade EDTA. Specific strains include Agrobacterium radiobacter ATCC 55002[29] and the sub-branches of Proteobacteria like BNC1, BNC2 [30] and strain DSM 9103.[31] The three strains share similar properties of aerobic respiration and are classified as gram-negative bacteria. Unlike photolysis, the chelated species is not exclusive to Fe(III) in order to be degraded. Rather, each strain uniquely consumes varying metal-EDTA complexes through several enzymatic pathways. Agrobacterium radiobacter only degrades Fe(III) EDTA [30] while BNC1 and DSM 9103 are not capable of degrading Fe(III) EDTA and are more suited for CaEDTA, BaEDTA, MgEDTA and MnEDTA.[32] EDTA complexes require dissociation before degradation .

Alternatives

Interest in environmental safety has brought up concerns about biodegradability in aminopolycarboxylates such as EDTA. For example, under the 28-day EN ISO 7827 test Austrian paper and pulp industries must use chelating agents that have biodegradation levels over 70% or 80% (after 28 days).[33] An increased interest in safety has led to the development and research of alternative chelating ligands which can still bind strongly to metal ions but also have a higher biodegradability and a lower content of nitrogen.[33]

Iminodisuccinic acid (IDS)

Commercially used since 1998, iminodisuccinic (IDS) acid biodegrades about 80% after only 7 days. IDS binds to calcium exceptionally well and forms stable compounds with other heavy metal ions. In addition to having a lower toxicity after chelation, the production of IDS is environment-friendly.[33] Specifically, IDS is degraded through the use of IDS-epimerase and C-N lyase found in Agrobacterium tumefaciens (BY6) which can be harvested on a large scale. Additionally, the reactions catalyzed by both enzymes do not require any cofactors and can thus be applied directly.[34]

Polyaspartic acid

Polyaspartic acid, like IDS binds to calcium and other heavy metal ions. It has a higher value of 7.2 meq/g than does EDTA, which only has 6.0 meq/g.[33] While it has a higher theoretical capacity, in practical applications it exhibits low efficiency in lower ion concentration solutions. It has many practical applications including corrosion inhibitors, waste water additives, and agricultural polymers. A Polyaspartic acid-based laundry detergent was the first laundry detergent in the world to receive the EU flower ecolabel.[33]

Ethylenediamine-N,N?-disuccinic acid (EDDS)

As a structural isomer of EDTA, ethylenediamine-N,N?-disuccinic acid can exist as three isomers: (S,S), (R,S)/(S,R) and (R,R), but only the S,S-isomer is readily biodegradable. EDDS exhibits a surprisingly high rate biodegradation at 83% in 20 days. Biodegradation rates also varies the different metal ions chelated. For example, the complexes of lead and zinc with EDDS have relatively the same stability but the lead complex is biodegrades more efficiently than the zinc complex.[33] As of 2002, EDDS has been commercially prominent in Europe on a large scale with an estimated demand rate increase of about 15% each year.[33]

Methylglycinediacetic acid

Methylglycinediacetic acid (MGDA) is produced from glycine.[33] MGDA has a high rate of biodegradation >68%, but unlike many other chelating agents can degrade without the assistance of adapted bacteria. Additionally, unlike EDDS or IDS, MGDA can withstand higher temperatures while maintaining a high stability as well as the entire pH range. As a result, the chelating strength of MGDA is stronger than many commercial chelating agents.[33]

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